TiN coatings on nickel and electroless nickel (EN) plated mild steel

TiN films were deposited by reactive d.c. magnetron sputtering on a mild st eel substrate. Because of the presence of inherent porosity and internal st resses TiN coatings cannot be relied upon to give corrosion protection, als o TiN coatings need a good support. Nickel and electroless nickel (EN) were deposited by plating as interlayers below the TiN coatings to study compar atively the effect of these interlayers in overcoming the two limitations. Nickel was electrodeposited using Watt's bath while EN (electroless nickel) was deposited using the acidic hypophosphite bath. Three different thickne sses viz. 2, 4 and 10 mu m were deposited to study their influence on TiN c oatings. The corrosion behaviours of TiN coated mild steel samples with and without the interlayers were studied using the potentiodynamic measurement technique in deaerated 1N H2SO4 electrolyte. It was observed that TiN coat ings on mild steel without any interlayer almost mimic the behaviour of the substrate, while the presence of any of the two interlayers showed a consi derable improvement in the corrosion resistance. Microhardness measurements revealed that the apparent surface hardness of T iN coatings on mild steel increased to 1484 HK25 from about 1000 HK25 when employed with EN interlayer, while electroplated nickel as an interlayer in creased the hardness to a lesser extent (1289 HK25). X-Ray diffraction analysis has shown that the TiN phase had preferred orien tations, mainly TiN(200) and TiN(111). Interlayer nickel showed the presenc e of mainly Ni(200) and Ni(111) peaks while EN was amorphous in structure. The crystalline phases Ni3P and Ni were evident when TiN top-coat was obtai ned by d.c. magnetron sputtering on EN plated mild steel samples.